Radio-frequency identification tags (or transponders) require a reference frequency for a number of purposes. An RFID reader transmits RF power to RFID tags. RFID tags modulate reflected RF power to transmit data back to an RFID reader. The reflected RF is called ‘backscatter,’ and the link from the tag back to the reader is typically referred to as the ‘backscatter link. The backscatter modulation of course requires a backscatter frequency to which the relevant RFID reader is sensitive. Furthermore, backscatter communications may be subject to regulatory restrictions, and may need to be compliant with one or more RFID communications specifications or standards. An RFID tag also requires a demodulation frequency so as to enable a demodulator within the RFID tag to demodulate received radio-frequency signals, and decode data contained therein. RFID tags also need to generate internal clock signals to clock various functional units that may be included within the RFID tag.
With a view to generating the above-identified frequency and clock signals within an RFID tag, the RFID tag is typically equipped with an oscillator that generates the reference frequency. Three prior art mechanisms for providing such a reference frequency are discussed below. FIG. 1 is a schematic illustration of a first prior art oscillator arrangement 10 in which an oscillator 12 is coupled to a crystal 14 in order to provide a precise local reference frequency. Alternatively, the oscillator 12 may be coupled to an L-C tank or electron mobility-based reference in order to provide the precise local reference frequency. A disadvantage of such arrangements is that they tend to be bulky, and high-power consumers.
A second manner in which it is known to provide a reference frequency within an RFID chip is to provide a phase-locked loop (PLL) arrangement, such as that illustrated by the schematic diagram of FIG. 2. Specifically, the phase-locked loop arrangement 16 of FIG. 2 is shown to include a phase detector 18 that is coupled to receive a reference frequency 20 and the oscillator output, compare them, and to provide a reference signal 22 to an oscillator 24. The disadvantages of the phase-locked loop arrangement 16 shown in FIG. 2 include the required provision of a reference frequency, a long start-up time, the provision of extra power for the phase detector 18, as well as the extra chip area requirements for provision of the phase detector 18. A similar function can also be done with a frequency detector, and a frequency-locked loop.
A third prior art arrangement 26 to provide a reference frequency within an RFID tag is illustrated by the schematic diagram of FIG. 3. Specifically, a trimming arrangement 28 comprising a combination of resistors, capacitors and inductors (or fuses or resistors that may be laser-cut) provide a reference signal 22 (e.g., a current reference signal Iref) to an oscillator 30. Among the disadvantages of this arrangement are that the trimming arrangement may be expensive to build, and the configuration of the trimming arrangement 28 is permanent (i.e., the oscillator 30 cannot be dynamically calibrated).
FIGS. 4 and 5 are diagrammatic representations of a prior art RFID system 32 including an RFID tag 34 that is interrogated by, and responds to, an RFID reader 36 utilizing a radio-frequency forward link and a backscatter return link. The RFID tag 34 is shown to provide a signal received from the RFID reader 36, via the radio-frequency forward link, to a demodulator 38, which recovers a timing (or clock) signal 40. The recovered clock signal 40 is utilized to generate a digital calibration value 44, which is stored in a volatile register 42. The volatile register 42 in turn provides the digital calibration value 44 to a digitally-controlled oscillator (DCO) 46. The digitally-controlled oscillator 46 outputs a demodulator clock signal 48.
FIG. 5 illustrates the oscillator 46 of the RFID tag 34, again calibrated utilizing a digital calibration value 44 provided to the oscillator 46 from the volatile register 42. The oscillator 46 generates a modulator clock signal 52 to a modulator 50, the modulator 50 utilizing the modulator clock signal 52 to backscatter modulate communications transmitted via the backscatter return link to the RFID reader 36.
In summary, it will be appreciated that, on start-up, the RFID reader 36 sends a radio-frequency forward link signal to the RFID tag 34, which extracts a timing (or clock) signal 40 from the received signal to calibrate the oscillator 46, this recovered timing signal 40 being communicated to the oscillator 46 via the register 42. During backscatter communications, the calibration is held by the oscillator 46, which is in turn utilized to drive the modulator 50.
Accordingly, in the prior art system shown in FIGS. 4 and 5, the recovered timing signal 40 is stored within a volatile register that is utilized to calibrate the oscillator. However, a clock recovery operation is required by the demodulator 38 upon each power-up event, which may negatively impact the performance of the RFID tag 34.
U.S. Pat. No. 5,583,819, entitled “Apparatus and Method of Use of Radiofrequency Identification Tags”, to Bruce B. Roesner and Ronald M. Ames, discloses an RFID tag in which a reference signal is initially generated by comparing an incoming standard signal, and placing it in a temporary or permanent storage within the RFID tag. Signals arriving later are then compared to the captured standard, and variations from the captured standards are detected to allow for decoding of the data. Specifically, a microprocessor is described as providing a correction signal to a memory, the correction signal then being stored within the memory as a correction value for use in subsequent operation of the RFID tag, or at least until the correction value is updated. The memory is described as possibly being a non-volatile memory to allow calibration information to be permanently stored, so that reconfiguration of the internal oscillator is not required each time the RFID tag is powered up.
In the system described by Roesner, the calibration of the oscillator is nonetheless dependent upon an initial extraction or recovery of timing from a received radio-frequency signal.